Re: [RFC v2] virtio: support packed ring

Tiwei Bie
2018-04-18T01:17:57Z

On Tue, Apr 17, 2018 at 06:54:51PM +0300, Michael S. Tsirkin wrote:
> On Tue, Apr 17, 2018 at 10:56:26PM +0800, Tiwei Bie wrote:
> > On Tue, Apr 17, 2018 at 05:04:59PM +0300, Michael S. Tsirkin wrote:
> > > On Tue, Apr 17, 2018 at 08:47:16PM +0800, Tiwei Bie wrote:
> > > > On Tue, Apr 17, 2018 at 03:17:41PM +0300, Michael S. Tsirkin wrote:
> > > > > On Tue, Apr 17, 2018 at 10:51:33AM +0800, Tiwei Bie wrote:
> > > > > > On Tue, Apr 17, 2018 at 10:11:58AM +0800, Jason Wang wrote:
> > > > > > > On 2018年04月13日 15:15, Tiwei Bie wrote:
> > > > > > > > On Fri, Apr 13, 2018 at 12:30:24PM +0800, Jason Wang wrote:
> > > > > > > > > On 2018年04月01日 22:12, Tiwei Bie wrote:
> > > > > > [...]
> > > > > > > > > > +static int detach_buf_packed(struct vring_virtqueue *vq, unsigned int head,
> > > > > > > > > > + void **ctx)
> > > > > > > > > > +{
> > > > > > > > > > + struct vring_packed_desc *desc;
> > > > > > > > > > + unsigned int i, j;
> > > > > > > > > > +
> > > > > > > > > > + /* Clear data ptr. */
> > > > > > > > > > + vq->desc_state[head].data = NULL;
> > > > > > > > > > +
> > > > > > > > > > + i = head;
> > > > > > > > > > +
> > > > > > > > > > + for (j = 0; j < vq->desc_state[head].num; j++) {
> > > > > > > > > > + desc = &vq->vring_packed.desc[i];
> > > > > > > > > > + vring_unmap_one_packed(vq, desc);
> > > > > > > > > > + desc->flags = 0x0;
> > > > > > > > > Looks like this is unnecessary.
> > > > > > > > It's safer to zero it. If we don't zero it, after we
> > > > > > > > call virtqueue_detach_unused_buf_packed() which calls
> > > > > > > > this function, the desc is still available to the
> > > > > > > > device.
> > > > > > >
> > > > > > > Well detach_unused_buf_packed() should be called after device is stopped,
> > > > > > > otherwise even if you try to clear, there will still be a window that device
> > > > > > > may use it.
> > > > > >
> > > > > > This is not about whether the device has been stopped or
> > > > > > not. We don't have other places to re-initialize the ring
> > > > > > descriptors and wrap_counter. So they need to be set to
> > > > > > the correct values when doing detach_unused_buf.
> > > > > >
> > > > > > Best regards,
> > > > > > Tiwei Bie
> > > > >
> > > > > find vqs is the time to do it.
> > > >
> > > > The .find_vqs() will call .setup_vq() which will eventually
> > > > call vring_create_virtqueue(). It's a different case. Here
> > > > we're talking about re-initializing the descs and updating
> > > > the wrap counter when detaching the unused descs (In this
> > > > case, split ring just needs to decrease vring.avail->idx).
> > > >
> > > > Best regards,
> > > > Tiwei Bie
> > >
> > > There's no requirement that virtqueue_detach_unused_buf re-initializes
> > > the descs. It happens on cleanup path just before drivers delete the
> > > vqs.
> >
> > Cool, I wasn't aware of it. I saw split ring decrease
> > vring.avail->idx after detaching an unused desc, so I
> > thought detaching unused desc also needs to make sure
> > that the ring state will be updated correspondingly.
>
>
> Hmm. You are right. Seems to be out console driver being out of spec.
> Will have to look at how to fix that :(
>
> It was done here:
>
> Commit b3258ff1d6086bd2b9eeb556844a868ad7d49bc8
> Author: Amit Shah <amit.shah@redhat.com>
> Date: Wed Mar 16 19:12:10 2011 +0530
>
> virtio: Decrement avail idx on buffer detach
>
> When detaching a buffer from a vq, the avail.idx value should be
> decremented as well.
>
> This was noticed by hot-unplugging a virtio console port and then
> plugging in a new one on the same number (re-using the vqs which were
> just 'disowned'). qemu reported
>
> 'Guest moved used index from 0 to 256'
>
> when any IO was attempted on the new port.
>
> CC: stable@kernel.org
> Reported-by: juzhang <juzhang@redhat.com>
> Signed-off-by: Amit Shah <amit.shah@redhat.com>
> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
>
> The spec is quite explicit though:
> A driver MUST NOT decrement the available idx on a live virtqueue (ie. there is no way to “unexpose”
> buffers).
>

Hmm.. Got it. Thanks!

Best regards,
Tiwei Bie

>
>
>
>
> > If there is no such requirement, do you think it's OK
> > to remove below two lines:
> >
> > vq->avail_idx_shadow--;
> > vq->vring.avail->idx = cpu_to_virtio16(_vq->vdev, vq->avail_idx_shadow);
> >
> > from virtqueue_detach_unused_buf(), and we could have
> > one generic function to handle both rings:
> >
> > void *virtqueue_detach_unused_buf(struct virtqueue *_vq)
> > {
> > struct vring_virtqueue *vq = to_vvq(_vq);
> > unsigned int num, i;
> > void *buf;
> >
> > START_USE(vq);
> >
> > num = vq->packed ? vq->vring_packed.num : vq->vring.num;
> >
> > for (i = 0; i < num; i++) {
> > if (!vq->desc_state[i].data)
> > continue;
> > /* detach_buf clears data, so grab it now. */
> > buf = vq->desc_state[i].data;
> > detach_buf(vq, i, NULL);
> > END_USE(vq);
> > return buf;
> > }
> > /* That should have freed everything. */
> > BUG_ON(vq->vq.num_free != num);
> >
> > END_USE(vq);
> > return NULL;
> > }
> >
> > Best regards,
> > Tiwei Bie
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